Properties of CdS:Al Films Deposited by Magnetron Sputtering

CdS:Al thin films with different doping concentrations were deposited by controlling deposition rate of CdS and Al targets in co-sputtering process. The morphological, structural, optical, and electrical properties of as-prepared CdS:Al films were investigated by X-ray diffraction (XRD), atomic force microscope (AFM), UV-visible absorption spectrum, and room temperature Hall-system. XRD patterns indicates that all the CdS:Al films are polycrystalline films with hexagonal wurtzite structure and have a preferential orientation in (002) direction. The uniform and compact films can effectively be formed on insulating substrates. It is worth noting that the average grain size increases with Al doping concentration. On the other hand, the surface root-mean-square (RMS) roughness also shows slightly increase. UV-visible transmittance spectra show that the band gap of CdS:Al film decreases slightly in the range of 2.42-2.46 eV with increasing Al doping concentration. The Hall measurements indicate that the effect of Al doping concentration on electrical property of CdS films is apparent. In comparison with the un-doped CdS thin films, the carrier concentration of CdS:Al films increases by three orders of magnitude when the Al concentration is higher than 3.8%, while the resistivity of which decreases by three orders of magnitude. The doping level of CdS films is improved by doping with Al atoms to enhance the built-in electric field, which may realize high open voltage (V-oc,) for CdTe thin film solar cells. The properties of CdS:Al film prepared by co-sputtering are suitable as a window layer of CdTe thin film solar cells.